The invention relates to a control device of an electromagnet comprising two power supply terminals, full-wave rectifying means connected to the power supply inputs, at least one coil connected in series with an electronic switch to the output of the rectifying means, means for controlling the electronic switch and a local control terminal designed to be connected to one of the power supply terminals by local control means able to take two states, respectively open and closed.
Conventionally the local control of an electromagnet, whether it involves a contactor or a coil of an electrical auxiliary (closing electromagnet XF or shunt release MX) of a circuit breaker, is formed by means of the pushbutton type. A programmable controller output is also sometimes used. In the latter case especially the presence of leakage currents may lead to spurious actuation of the electromagnet.
U.S. Pat. No. 5,394,018 describes a microprocessor-based contactor comprising means for distinguishing real AC control signals and stray input signals due to the presence of capacitors connected in parallel on the local control means. For this it comprises, between the local control means and the input, means enabling the offset existing between a stray input signal and a real input signal to be increased. Reading of the input signal is performed during this offset period. Such a system is complex and requires more components than an analog input (single dividing bridge).
The object of the invention is to achieve a device enabling this distinction to be made between real control signals and stray signals without presenting the shortcomings of the known contactor. This device must in addition operate in DC and at 400 Hz.
According to the invention this object is achieved by the fact that the device comprises a first resistive dividing bridge, connected to the output of the rectifying means and comprising a mid-point connected to a first input of the control means of the electronic switch, and a second resistive dividing bridge, connected between the local control terminal and an output terminal of the rectifying means and comprising a mid-point connected to a second input of the control means of the electronic switch, the control means comprising:
quasi-simultaneous sampling means for supplying associated samples of the first and second voltages applied respectively to the first and second inputs of the control means,
comparison means for comparison of a first quantity, representative of at least one non-null sample of the second voltage, and of a preset percentage of a second quantity, representative of associated samples of the first voltage,
and detection means for detecting the state of the local control means, the local control means being considered as being closed when the first quantity is greater than said percentage.
According to a development of the invention, the comparison means comprise means for comparing the first quantity to first and second percentages of the second quantity, the local control means being considered as being closed by the detection means when the first quantity is greater than or equal to the first percentage and as being open if it is lower than or equal to the second percentage, the second percentage being lower than the first percentage.
In a preferred embodiment, the local control means are considered by the detection means as not having changed state if the first quantity is comprised between the first and second percentages.
The first and second quantities are preferably formed respectively by the sum of a preset number of successive non-null samples of the second voltage and by the sum of the same number of associated samples of the first voltage.
The number of samples used for computation of the first and second quantity is preferably equal to or greater than 3, and for example 8. A decision concerning the state of the local control means is preferably taken after a preset number of successive comparisons leading to the same result.